A secondary battery which has ease with applicability for various product groups and good electric characteristics such as high energy density is universally applied not only to portable devices but also electric vehicles (EV), hybrid electric vehicles (HEV), energy storage systems or the like, which are driven by an electric source. The secondary battery has a primary advantage of greatly reducing the use of fossil fuels and a secondary advantage of generating no byproduct after the use of energy, and thus the secondary battery receives attention as a new energy source which is environment-friendly and promotes energy efficiency.
A battery pack applied to an electric vehicle or the like includes a plurality of cell assembly connected in series, each cell assembly having a plurality of unit cells, in order to obtain high output. In addition, the unit cell includes positive and negative electrode current collectors, a separator, active materials, an electrolyte or the like and is capable of being repeatedly charged and discharged by means of electrochemical reactions among the components.
Meanwhile, recently, as the necessity for a large-capacity structure to be utilized as an energy storage is increasing, the demand for a battery pack having a multi-module structure in which a plurality of modules are aggregated is also increasing.
In the battery pack of a multi-module structure, a plurality of secondary batteries is densely arranged in a narrow space, and thus it is important to easily emit heat generated from each battery module.
In order to cool the battery pack, an indirect water-cooling method is used.
The indirect water-cooling method is used for preventing a battery module from generating heat by using a heat sink with a cooling channel, and in general case, a single heat sink is coupled to a lower end of the battery module to absorb heat of the battery module.
FIGS. 1 and 2 are diagrams showing an existing heat sink applied for the indirect water-cooling method.
Referring to FIGS. 1 and 2, the existing heat sink 1 is coupled to a lower surface of a cell assembly 2 to cool the cell assembly 2. A cooling channel is formed in the heat sink 1, and a coolant inflow tube 3 and a coolant outlet tube 4 are also formed thereat. If a coolant flows into the coolant inflow tube 3, the coolant circulates along the cooling channel formed in the heat sink 1 to cool the cell assembly 2 and flows out through the coolant outlet tube 4.
However, the existing heat sink using the indirect water-cooling method has a problem in that its structure increases the entire height of a battery pack. In other words, since the upper surface of the existing heat sink 1 is closely coupled to a lower surface of the cell assembly 2, the entire height of the battery pack increases.
In addition, if the upper surface of the heat sink 1 is coupled to the lower surface of the cell assembly 2, a differential pressure increases at the coolant inflow tube 3 and the coolant outlet tube 4. In detail, since the heat sink 1 is closely coupled to the lower surface of the cell assembly 2, the coolant inflow tube 3 and the coolant outlet tube 4 are respectively bent twice, and due to such a bending structure, a differential pressure increases at the bent portions. In other words, the coolant inflow tube 3 and the coolant outlet tube 4 are firstly bent from a vertical direction to a horizontal direction and then secondly bent from the horizontal direction to the vertical direction, and due to such several bent portions, a differential pressure increases at the coolant inflow tube 3 and the coolant outlet tube 4.